(1) Field of the Invention
The present invention relates to a color image forming apparatus in which a final image is obtained by forming an image at the image forming station and transferring the formed image onto a print media being conveyed to the image forming station by print media conveying means. More specifically, the present invention is directed to an image forming apparatus of an electrophotographic type for forming color images such as a color copier, color printer etc.
(2) Description of the Prior Art
In recent years, color copiers have been developed to meet the demands for color recordings for office use.
One mechanism for such color copiers is of a tandem type, which comprises four photoreceptor drums arranged in parallel to each other, each drum being provided with a charger device, writing unit such as a LSU or the like, developing unit, transfer device, cleaning device arranged therearound. In this configuration, light reflected from a color original is separated into color components, each different color beam being supplied to a corresponding photoreceptor drum so that a corresponding latent image is formed thereon. The thus formed latent images on the photoreceptors are developed with toners of yellow, magenta, cyan and black, respectively, and the developed images are transferred in a superimposed manner onto printing paper which is supported on a transfer conveyor belt, thus producing a copy of the color image. When full color image forming is not needed, for example, in the case where a black monochrome image is formed, no toner image will be formed with the three photoreceptor drums of yellow, magenta and cyan and the black toner image is formed alone and then is transferred to the printing paper, thus producing a black monochrome image.
The method of driving the photoreceptor drums is disclosed in Japanese Patent Application Laid-Open Sho 61 No.156,161, in which all the photoreceptor drums are driven by a single driver source.
Development of color copiers of the tandem type provides a copy speed in the color copying copier, comparable to that in B/W monochrome copiers, but the price is still considerably high. When black monochrome copying is performed with a color copier of this type, the photoreceptor drums for colors which will not be used for the image forming, also are driven and hence worn out unnecessarily by the cleaning blades and the transfer conveyer belt. Further, when a black monochrome image is formed, the photoreceptor drums for colors also operate in contact with the transfer conveyer belt, which stains the belt with toner, causing degradation of image quality. Further, if the process speed is designed to be faster in order to enhance the speed of black monochrome copying, a large increase in the driving force is needed since the driver drives the photoreceptor drums for colors as well.